Pulse echo distance indicator



April 8, 1952 B. M. OLIVER 2,591,856

PULSE ECHO DISTANCE INDICATOR Filed Dec. 9, 1944 4 sheets-sheet 1 April 8, 1952 B. M. OLIVER 2,591,855

PULSE ECHO DISTANCE INDICATOR Filed Dec. 9. 1944 4 sheets-sheet 2 arm wmw ...33? 1 a NN MN geen QN L w nl April 8, 1952 B. M.'ol.|vER

PULSE ECHO DISTANCE INDICATOR 4 Sheets-Sheet 5 Filed Dec. 9,A 1944 NVENTOR @MOL/VER Z7 C MA? ATTORNEY April 8, 1952 B. M. OLIVER 2,591,856

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6, INPUT voLTAcE aF v9 (LEFT HALF) 26, uT-oFF L/Ne g/lvFL/r vaLTAcL oF v9 R/cHT HALF J vim k WW PLATE cuRREA/T 0F l (RIGHT HALF) V9 X270 K g TIME /N VE N TOR BM. OL/ VE R BY ATTORNEY Patented Apr. 8, 1952 PULSE Eeno- DISTANCE INDICATOR Bernard M. Oliver, New York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application-December 9, 1944, Serial No. 567,407

32Claims.

1 This invention relates to pulse-actuated circuits and more specifically to pulse reflection type image locating and distance measuring systems.

In a pulse reiiection type image locating. and distance measuring system (frequently called a radar), radio frequency pulses are directively emitted, reflections thereof are received. fromtobf jects upon which the emitted pulses-impinger and the reflection times for particular objectsare. determined to provide indications of the distances to these'objects. Electrical variations received at the receiving point constitute a repeatedrseries of impulses, each cycle. ofthe series. comprising, for example, a transmitted pulse andone orA more reflections or echo pulses interspersed-with and superposed on variations due tu noise.r The present invention in one of its primary aspectsrelates to the interpretation of the echoes or. infother words, the utilization of these echoes to determine the corresponding distances from the operating station to the particular objects giving rise to the echoes.

It is. an object of this invention to provide-a newv and improved pulse reflectiontype image locating and. distance measuringvsysteml which is simple and accurate.

It is another object of this invention .-to provide new and improved means for accurately indicating the time interval between-two time-displaced, relatively short pulses.-

In accordance with the invention, there is-provided a pulse reflection type image locating-- and distance measuring system comprising a transmitter, a transmitting antenna for emitting radio frequency pulses (called transmitted pulses or emitted pulses) produced by the transmitter, a receiving antenna, a receiver,` and a novel echo interpreter for indicatingl or measuring the distance to the nearest objector other selected one. The echo interpreter comprises two tubes cross-connected to form a monovibrator (a multivibrator circuit in which one tube is so biased. that only one cycle is delivered p er stimulus, or, in other Words, a non-free-running multivibrator circuit) and to which is fed-those pulses from the receiver which have passed through a cut-off tube which removes. practically all the noise. The monovibratorV is initiatedby the transmitted pulse. and restored by the first echoand. thereby forms a pulse the duration of Whichissubstantially equal to the time-interval between the transmitted pulse and the .rst recel-ved echo.. Thispulse produced by the monovibrator is amplified and clipped to form a 2. square-topped pulse having the-same dura-tion: A. relatively long blocking pulse, starting at. the trailing edge of the square-topped pulse, is formed and isv fedback. to the cutoff-tub`e-to make this tube non-conducting to al1 echoes-after the first. Echoes after-the first one and before the next transmitted pulse, therefore, haye'no effecty on the monovibrator.` The seriesr of square-topped pulses is-applied to -a-l-ter which passes 1 only thefundamental frequency. of* the series,4 400- per second, forV example. Theoutput of the filter is amplified and thenrectified-- Asall of the square-topped pulses are-of thesame height, theareas of "the pulses are-proportional tothe lengths thereof. The rectified* fundamental component is, therefore, substantially proportional-tothe lengthof the monovibr'artor pulse and hence tothe range ofthe yobjectkforming the echo..

The inventionwill `bev more readily understood by referring to the followingy description-taken in connection with the accompanying drawings forming a part thereof in which:

Fig. 1 shows, in block diagram form,- a pulse. reflection type image locating and-'distancefmeasuring system in accordance-'with the invention;A

Figs. 2 and 3 are circ-uit diagrams-of portions of the system shown in Fig. 1; and

Figs. 4; 5, and 6 are'various graphical representationsto aid in understandingthe invention;

Referring more specifically to the drawings; Fig.l 1 shows, by way of example for illustrative purposes and in schematic diagram form, ai pulse reection type image locating and distance measuring system. This n'gureis notintendedto be a circuit diag-ram but merely a diagranrshow ing the arrangement of the various partsvofth'e system. In the arrangement of-Fig'. 1, artranse mitter I0l is provided to'supply'ultraehighfrequency pulse-modulated radio waves which" can be directed at a particular object orobjects'by means of an antenna Il. By Way of example. transmitter I0 can comprise an oscillator for providing a sinev wavehaving a suitable periodicitywhich4 can conveniently be 400 cycles p er second,v for example. 'This oscillator energizes the pulse generator ofA any one of severalsuit'f able types well known to the art. For. example; see United States Patent 2,117,752issued`May17, 1938 to L. R. Wrathall, which shows a generator which. provides. an energy pulse atV a .particular point of each cycle of the input wave provided .to it. The pulse. from the-pulse generator-.isapplled to a carrier generator and. modulator of. any suitable.- type and` the resultant pulsemodulated 3 waves are applied to the transmitting antenna II. Waves reected from one or more objects within the range of the transmitting antenna I l are received by a receiving antenna I2. The antennas II and I2 are of any suitable type; for example, they can be or the polystyrene polyrod type disclosed in an application of G. E. MuellenYSerial No. 469,284, led December 17, 1942 and which issued as Patent 2,425,366, on August 12, 1947. The reflected waves picked up by the receiving antenna I2 (and also the transmitted pulses) are applied to a receiver I3 of any suitable form wherein they are amplified, rectified and fed to the echo interpreter I4 of this invention. The input pulse wave to the circuit I4 (indicated in dashed lines in Fig. 1) is shown in Fig. 4 at line A. This pulse Wave comprises a transmitted pulse 266, and echo pulses 202, 203,etc. interspersed with spurious orl noise Variations. If desired, the output of the receiver .I3 can be applied to an oscilloscope for directvsual `indication of the echo pulses in addition-.to .being applied to the circuit I4. `,Reference .will now be made to Figs. 2 and 3 inxaddi-tion to Fig. l and Fig. 4 in describing the circuit I4. The output of the receiver I3 is connectedto input terminals 20 and 2l of the tube-VI,- a resistor 22 being connected across the input4 terminals. vThe terminal 20 is connected tov-the control grid of the tube VI, while the cathode of this tube is grounded. The suppressor grid is connected to the cathode and the screen grid is connected through the resistor 23 toi-a terminal -24 which is-connected to the positive .polefof a source 25 which has been represented as a -battery but which may be any suitable source of direct. potential. The terminal 24 hasy been represented by short dashed lines in order to indicate that individual filters may be provided as required. The screen grid is also ley-passed to ground through the condenser 26; |T [Ihe .anode is connected to the terminal 24 through the anode resistor 21 and the equalizing inductance member 28. The anode of the tube VI is connected through the coupling condenser 30.2 to the `control grid of the tube V2, the leak resistor 3| being connected between this grid and ground.' Bias for the tube V2 is supplied by meansof the parallel-connected resistance member.32 and capacity member 33 which are connected. between the cathode and ground. The cathode is placed at a positive potential with respect to `ground by means of the resistors 34 and. 35,'acting with `the resistor 32 to form a potential-dividing potentiometer. The suppressor grid of the tube V2 is connected to the cathode and the screen grid is connected through the resistor..34 to the terminal 24 and through the by-pass condenser 36 to ground. The anode is connected to the terminal 24 through the anode resistor 31 and the equalizing inductance member 3B. The tube V2 is biased below cut-ofi" so that only rthe peaks of the signals applied to the input circuit of this tube cause current to ow in the output circuit thereof. The input .voltage of the tube V2 is indicated in line B of Fig. 4 and the output voltage thereof is shown in line C of vthis figure. These Wave shapes Will be explained hereinafter.

The output voltage of the tube V2 is applied by means of the coupling condenser 4) to the screen grids. of the tubes V3 and V4. These grids are connected through the resistance member 4I to the terminal 24 to provide a positive potential thereon. The cathode of the tube V3 is connected to4 ground while the cathode of the tube V4 is connected to ground through the parallelconnected resistance member 42 and capacity member 43. In the absence of a pulse applied to the screen grids, only the tube V3 is conducting as the tube V4 is biased beyond cut-off. The suppressor grids of the tubes V3 and V4 are connected to their respective cathodes. The control grid of the tube V3 is connected through the coupling condenser 44 to the anode of the tube V4, while the control grid of the tube V4 is connected through the coupling condenser 45 to the anode of the tube V3. A leak resistor 46 is connected between the control grid of the tube V3 and ground while resistor 41 is connected between the control grid of the tube V4 and ground. The anode of the tube V3 is connected to the terminal 24 through the resistor 48 and equalizing inductance member 49, while the plate of the tube V4 is connected to the terminal 24 through resistance member 50 and inductance member 5 I Thel cathode of the tube V4 is placed at a positive potential by means of the resistance member 52 acting with the resistance 42 as a voltage dividing potentiometer. The tubes V3 and V4 serve to produce on the plate of the tube V4 a wave of the type shown in Fig. 4, line E which is applied to the control grid of the tube V5 through the coupling condenser 53.

The control grid of the tube V5 is connected to ground through a resistor 54, while the suppressor grid is connected to the cathode and to ground through the resistor 55. The screen grid is connected to the terminal 24 through the resistor 56 and is by-passed to ground by means of the condenser 51. The anode of the tube V5 is `connected to the terminal 24 through the resistance member 58 and the equalizing inductance member 59.

The anode of the tube V5 is connected to the control grid of the tube V8 through coupling condenser 60 and also through this coupling condenser to the anode of the tube V6, which is shown in the drawing as a pentode connected as a diode, the three grids being connected together and to the anode of the tube. The cathode of the tube V6 is connected to the tap 6I of the potentiometer 62 which is connected between ground and one terminal of the resistor 63, the other terminal of which is connected to the ter-v minal 24. A voltage regulating tube V1 is connected across the resistance 62. The cathode of the tube V6 is also connected to ground through the condenser 64.

The control grid of the tube V8 is connected to ground through the leak resistor 65 and the cathode is connected to ground through the biasing resistor 66. The suppressor grid is con# nected to the cathode and the screen grid is connected to the terminal 24 through the resistor 61. The anode of the tube V8 is connected to terminal 24 through the resistor 68 and is connected to the terminal 10 (leading to the part of the circuit I4 shown in Fig. 3) through coupling condenser 69. A resistor 12 is connected between terminal 16 and the grounded i terminalll.

The cathodev of the tube V8 is connected by means of a connection 'I3 and condenser 14 of small capacity to the grid of the left half of the tube V9. This grid is connected to ground through the resistor 15. The cathode of the tube V9 (left half) is connected directly to ground while the anode of this half` is connected to the terminal 24 through the anode resistor 1B. lThis anode is also connected through the coupling atenerse condenser 1-1 to the-grid of the righthalf'- of thel tube V9.- This' latter grid isconnected: to ground through the leak resistance 18. The cathode of this half is connectedv directly to ground and the anode thereof is connected to the terminal 24 through the resistance member 21 andthe inductance member 28. This anode is also connected'to the tube V2 through the coupling condenser 30. The grid Voltage-of the tubeVZ'is shown in Fig. 4, line-B. As shown in this figure and as will be explained morefu'll'y below, only the rst echo pulse 2 I 6 is large enough to` have any effect on' the operationvofmonovibrators V3- and V4, the action of the-tube V9 serving to squelch thea action of the othersecho pulses 2I2', 2I3,4 etc. by lowering them belowthe cut-ou" line, hence, the name for this tubein Fig. I.

The output terminals andY v1I ofv the tube V8 areconnected to a 400-cycle--lter comprising-'a condenserl 80 connected acrossY the terminals, an inductance memberl 8| connected to Aterminal 10, transformer 83, and capacity 85; Theter'# minal of the inductan'ce member 811 not connected to the terminal 10 is connected to one terminal of primary winding 82VA of the transformer 83, the other terminal of this Winding being connected to the terminal 1I.` Secondary winding- 84-'of the transformer 8f3 is shunted'by condenser 85 and one terminal of this winding is connected to ground. The ungrounded ter'- minal of the winding 84 is connected; through the parallel-connected condenser member 80 and resistance member 81 to the grid of the'amplifier tube VIO', the cathode of which is connected to'fgroundthrough the resistor 88. The'grid'of this tube is at a positive potential'determined by the yvalues of the resistor 89'connectedto the positive terminal 24 ofthe source 25"and'resis tor 81. The anode of the tube VIO is connected through the resistance 90 and the `variable'resistance 9I to the terminal 24. The output voltage-of the amplifier VI`0 is applied to the control gridv of thetube VII' through a couplingicon'- denser 92, this grid being connected to ground through` a leak resistor 93. VII is used as a triode as the suppressor grid, screen grid, and theanod'e ofthe tube VII are connected together and' to the terminal 2.4 of the source 25 through'the resistor` 94 andthe resistor 95, the common terminal of these two resistors being connected to ground through the lter condenserv 96. The cathode of the tube VII is connected to ground through condenser 91 which is shunted by a resistance 98. This cathode is also connectedto the two cathodesv of the double diode tubeVIZ and through the resistance members 99,v |00 and I0'I tothe control' grid of the tube VI3., Thev com.- mon terminalof' the resistors 99 and I0`0 i`s. connected to ground' through condenser I`02' and through a switch |03 to the anode of the left half of the double diode tube VI2 while the common terminal of resistors I 00 and IOI is connected to ground through the'conden'ser |04' and throughV a switch I05'to the anode of the'right half of thev double diode ltube VI'2. The resistancemembers 99 and |00 and the condensers. 91, I02-and |04 serve-asY a smoothing ilter.l

The cathode of the tubev VI3 is connectedv to ground through by`pass condenser IUS land is also connected directly to an output terminal I'I'0 which forms' a pair ofterminalswith a grounded terminal |20. The cathode is also connected through resistances'Y III and II2 to an output terminal II3 ot" an external'range meter (not shown) and through a resistance |l4 and. an internal range meterl I5 to anA innertap. IIB of aresistor II1. The tap IIB is alsoconnected to a: terminal II8'of the external range meter. One terminal of theresistance II1 is. grounded and the other' terminal is4 connected through. a voltagefiregulating tube VI4 and the resistor 95 to the-positive terminal 24 of' thesource 25'.. The anode of' the tube VI3 is` also connected. to/this terminal through a resistor I I9.

The operation of the circuit shown in Figs. 2 and k3i will nowL be described, reference also being made tol the graphical representations of Figs; 4 5`and 6; Voltage waves of the type-shown in lineA'of Fig. 4 and comprising transmitted pulse'200 and echo pulses 20I, 202, 203, etc., are'appliedvto -the'input of the tube VI which arnpli'fles. and. phaseinverts these signals. VThe output: of the-tube VI and the output of the tube V9, theaction offwhich will be explained below,r are applied tov the grid of the tube V2', the combined wave being shown in Fig. 4, line B; Pulse 2I0- of kthis combined wave corresponds to. the transmitted pulse 200 and the pulses 2II, 2 I2 and 2I3l correspond to the echoes 20|., 202 and- 203. Were it not for the action ofv the squelch tube V9, the pulses 2I-2 and 2I3l would be at thev same general level as the pulse 2I I. Asthe anode ofthe tube V2 is connected tto thev screen gridsof the tubes V3 and V4, both of these screen grids are drivennegativey upon the appearance of each pulse 22| (see Fig'. 4,'y line- C) in the output of the tube V2. The pulses 2I2 and 2I3, being below the cut-OITline do not produce corresponding pulses inthe'output wave of V2. The plates and control` grids` of the tubes V3 and V4 are crossconnectedto form a monovibrator. Before-'the appearance of a pulse 220 in the output of the tube V2, the tube V3 is conducting while the tube V4 isibiased beyond its cut-01T point. When the pulsev 220, corresponding in position to the transmitted pulse 200, is applied to the screen grids of the tubes V?.v and V4, these screen grids are. both dri-ven negative cutting off the tube V3 `so. thatlboth tubes are. non-conducting for the-duration of 'the pulse 220. The plate voltage curve* 2301- for the tube V3 is ,shown in line D ofl Fig. 4 while the plate voltage curve of the tube V4 is shown in line E of Fig. 4, both curves 230 and; 235-being at maximum voltage for the duration-of the pulse 220. When the tube-V3 is. cutv on', the positive pulse 230 is applied to the control grid of the tube V4 throughthe coupling, condenser 45 but this latter tube does notrbecome :conducting during the time that the negative. pulse 220 isy applied to the screen grid of. the `tube V4. As soon as the pulse 22Illterminates however, the screen grids of both tubes V3 and V4Y are` driven in a positive direction, allowing` the, tube V4 to conduct and decrease its plate Voltage sharply as shown in line E of Fig.. 4". This causes a negative pulse to vbe applied. to the. control grid of the tube V3 through the-couplingY condenser 44 thus keeping the tube V3-cut off. After the lrst'sharp dip, plate voltageof the tubeV4 begins to rise due to the leakage'L of the charge on the condenser 44 through the resistance 46. If no further pulse appears for several microseconds after the pulse 220,.the grid voltageof the tube V3 may rise to such an extent that' the circuit will snapback after the manner of a monovibrator into its resting phase, .thatis, into its condition beiore the occurrence ofthe pulse 220. Should an echo pulse appearbefore thishappens, however, the snap- 7 back will be initiated by the echo pulse. Such a pulse is represented by the pulse 22I in line C of Fig. 4. This pulse again causes the screen grid of the tubes V3 and V4 to go negative, cutting oil the tube V4 and causing a positive pulse to be applied to the control grid of the tube V3 but as this tube is cut-ofi' by the negative pulse on its screen grid it cannot become conducting until pulse 22I terminates. When this latter even takes place, the tube V3 becomes conducting and the plate voltage thereof drops sharply as shownin line D of Fig. 4. The plate voltage of the tube V4 rises to its maximum or B+ potential coincidentally with the -leading edge of Ythe pulseV 22|. The monovibrator comprising the tubes V3 and V4 is then in the resting phase again and it stays in this phase indeiiriitely or until another pulse appears in its input circuit because even though the control grid of the tube V4 leaks clear up to ground potential, tube V4 will still be cut on" by the bias on the cathode produced by the potentiomf eter resistors 42 and 52 connected between the terminal 24 of the source 25 and ground. It will be seen that the length of time the monos vibrator remains in its short phase (as distinguished from its resting phase) and therefore the length of time the plate of the tube V4' is not at its B-I- potential is equal to the time between the trailing edge of the transmitted pulse 200 and the leading edge of the rst echo 20I as indicated by the dimension At in line E of Fig. 4. At therefore is equal to the pulse delay minus the width of the transmitted pulse. The negative pulse 235 from the plate of the tube V4 is applied to the control grid of the tube V5 and drives its grid past itscut-off point. The bottom irregularities are thus clipped off and in the plate circuit of the tube V5 there is produced a rectangular pulse 240 as shown in line F of Fig. 4. The potential of the top of this pulse is fixed on the right-hand side of the blocking condenser 60 by the diode V6 whose plate can never go appreciably more positive than the cathode the potential Va of which is, in turn, xed by the voltage regulating tube V'I and the setting of the tap 6I on the potentiometer 62. This action limits the top of the .pulse to a constant potential shown in line F of Fig. 4 as the voltage Va. The cut-oil potential of tube V8 is constant also so that the charge which flows through V8 will be proportional to Atand therefore to the delay, minus a constant. The cathode of the tube V0, being unbypassed, will accompany the grid of this tube on its positive excursions to produce a pulse 250 as shown in line H of Fig. 4. In addition to stabilizing the vgain of tube V8, this cathode provides a low impedance source of a replica of the output p-ulse which is used to generate a blanking pulse or wave in the output of the tube V9. The cathode voltage wave 250 shown in line H of Fig. 4 is reshaped by the small capacity 'I4 and the grid-to-cathode resistance of the left-hand side of the tube VS and the leak resistance 15 to form voltage waves 260 shown in line I of Fig. 4. The shaping of the voltage wave 250 is unsymmetrical since the grid resistance of the left-hand side of the tube VS is much less than the resistance of the member 75.

The leading edge of the Wave 250 drives thegrid of V9 (left half) positive and the grid current quickly charges the small condenser 14. The trailing edge of pulse 250 then drives the grid of V9 (left half) negative, but now condenser I4 discharges very slowly through the leak 15.

Thus, the exponential transient 26I in the negative direction is a thousand or more times longer than the positive exponential 262 and cuts the left-hand section of the tube V9 oi for several hundred microseconds. By way of example, the length of the pulse 250 may be of the order of 50 microseconds or less and the pulses 200 may be ofthe order of 2500 microseconds apart. This cutting oi of the first section of the tube V9 makes the second or right-half section conduct a large current and cut off the tube V2 so far that the pulses applied to the tube VI after the first echo pulse 2II, that is other and more distant echo -pulses 2l2 and 2I3, for example, cannot cause conduction in V2. This action is shown in lines J, K and B of Fig. 4. The rst of these lines (J) shows the input voltage 265 of the rightdhalf of the tube `V9 which comprises a small negative exponential 266 corresponding to the exponential 262 in the wave 260 and a relatively long positive pulse starting with the beginning ofthe negative pulse 26| in the wave 260 and terminating substantially at the time the pulse 26I reaches the cut-off line of the tube V9. The pulse 266, being below the cut-off line of the right half of the tube V9, does not appear in the plate current curve 210 shownin line K. A plate voltage wave corresponding to the plate current wave but in the opposite direction is applied to the control grid of the tube V2 along with the input to the tube V2 from the tube VI to produce the wave shape shown in line `B of this figure. As shown in line B of Fig. 4, echoes 2 I2 and 2 I3 later than the first one 2II used to restore the monovibrator or snap circuit comprising the tubes V3 and V4, therefore., are blanked out and have no effect on the monovibrator. By the time the blanking pulse on the grid of the tube V2 has vanished, so have all echoes and the circuit is ready for the next transmitted pulse 200. In this fashion the circuit is made to respond only to thev transmitted pulse and the nearest echo.

The output of the tube V8, as shown in line G of Fig. 4, consists of a negative pulse 245 of fixed amplitude and of a length proportional to the delay of the first echo pulse 20| (minus the pulse width). The area of this output pulse, therefore, is proportional to the delay and since the duty cycle is so short (assuming that the transmitted pulses 200 are approximately 2500 microseconds apart), the 40o-cycle component present is proportional to their area. If, therefore, the

.40G-cycle component is extracted or filtered by means of the lter circuit comprising the con- 'denser 80 and the inductance member 8|, am-

.plied in the tube VIO, rectied in the tube VII and then smoothed in the output amplifier VI3 having a cathode follower output, a voltage is produced which is proportional to the area of the pulse 245 and hence is proportional to the range. the internal range .meter II5 and through the external range meter (not shown) connected between the terminals II3 and II8. These meters are calibrated to read range in yards and thus give an accurate indication of the range to the target producing the first echo 20 I The doublediode tube VI2 is added to make the transient response of the filter, comprising the condenser elements 91, |02 and |04 and the resistance ele- This is donev to echo. Should the rst echo fade for a short time, the snap circuit comprising the tubes V3 and V4 will not be restored to its original phase at the proper time 'but will continue in the short This direct current output flows through' v.9 phase until the next more distant echo, if any, or until its time constants restore it automatically to the long or resting phase. This means that Vduring those cycles when the-desired echo is vline in Fig. 6. 'I'he timescale of Fig. 6 is quite different rfrom that used in lines A to K, inclusive, of `Fig. Yi as the time between two transmitted pulses 200 in Fig. 4 is approximately 2500 microseconds whereas the over-all time period for the curve shown in Fig. 6 is many seconds. The output voltage of the vtubeVI l'during'the series of fades is shown by the solid line portions of Fig. 6, the voltage corresponding to the true range being shown by the dotted line. Since the minimum values of the solid curve are always the correct ones sudden increases from these minimum values should be ignored in the output. The tube V|2 allows the Voltage on the grid of the tube V|3 to climb slowly during the fades (shown in dashed lines) but fast enough to take care of true range changes, but snaps the potential back down to the minimum value when the pulse reappears by conducting at this time and shorting the resistances 99 and |00 which are very large, of the order of 5 megohms, and which form with the condensers 91, |02 and |04 the long time constant of the rise.

The switches |03 and |05 are provided for removing the tube V l2 from the circuit if it is desired to do so. The current in the tube VM, which is a voltage regulator tube, is used to buck some of the current cf the tube V|3 in order to produce zero reading at zero range on both of the meters. The zero setting is obtained by varying the position of the tap IIS on the resistor in series with the tube VM. By controlling the amplification of the stage including tube VID (by adjusting the setting of the resistor 9|) the scale of the internal range meter is adjusted. An additional adjustment for the scale of the external or remote indicating meter to make the two meters agree is accomplished by adjusting the resistor ||2. are voltmeters and they are connected in parallel.

-The outputl terminals H9 and |20 may go to any other form of potential indicator or nullsetting device, such as a servo-comparator.

If it is desired to obtain the range to targets other than the nearest one, a blanking circuit can be added which supresses all video signals for a certain time after each transmitted pulse. The rst echo beyond this excluded range will then restore the monovibrator, while nearer echoes will have no effect. Obviously it might be desirable to make the duration of this blanked interval variable and automatically adjusted to a value slightly less than the delay time of the desired echo. A blanking tube connected into the circuit in the same way as V9 but arranged to operate olf the transmitted pulse can be used to accomplish this action.

Circuit constants for an operative arrange- Both of the meters ment constructed in accordance with this invention lhave Vbeen shown on the drawings, but it is obvious Athatthe invention is notlimited tojthe useof elements hav-ing these particular circuit constants. n

While there have been` shown and particularly described a certain embodiment of the invention and a method of operation embraced therein for the ,purpose of explaining its principles and showing its applications, it will be obvious `to those skilled in the art that many Ymodifications and variations are possible and it is therefore intended to cover all such modifications and variations 'as fall within the scope of the invention which is defined in the claims.

`What is claimed is:

1. A circuit for measuring the time interval between one vof a .series of pulses emitted-'at a given repetition. frequency and .a first echopro- .duced by `said .pulse comprising means. forproducing a square-top pulse which is started substantially coincident with said emitted pulse landlis terminated vsubstantially coincident with' said rst echo, means for extracting from said squaretop pulse a frequency component corresponding to the repetition frequency of said emitted pulses, and means for rectifying and filtering the voltage of said frequency to produce a direct voltage, representative of the intensity of said frequency component, which is a measure of the time interval between the emitted pulse and the first echo.

2. In combination, two electron discharge devices each comprising an anode, a cathode and a first and a second grid, means for connecting the first grid of each of said tubes to the anode of the other of said tubes, means for connecting a biasing potential between the first grid of the first one only of said tubes and the cathode thereof suicient to cause said tube to be normally cut off in the absence of pulses applied to the second grid, the second of said tubes being normally conducting, said means for coupling said first grid and said anode comprising time constant circuits, means for applying a series of negative pulses to the second grid of each of said tubes, whereby the one of said tubes which is not cut oi becomes non-conducting on the application of the iirst pulse in the series to the second grid thereof thereby applying a positive pulse to the first grid of the other tube which continues to be cut oi because of the negative pulse applied to the second grid thereof until the termination of the first pulse in the series whereupon it becomes conducting until the start of the next pulse.

3. In combination, means for producing a series of transmitted pulses, means for producing from each pulse in the first series a reflection or echo pulse but which reflections or echo pulses may fade from time to time, means for producing from the echo pulses and the transmitted pulses a series of square-topped pulses each having a duration corresponding to the time interval f between a transmitted pulse and its corresponding echo pulse and having a constant amplitude, said last-mentioned means being characterized in that when one of said echo pulses fades the square-topped pulse produced thereby is of much longer duration than the time interval between the transmitted pulse and its corresponding echo pulse, means including a rectifier and a filter circuit for producing a direct current intended to be representative of the duration of said square-topped pulses, but which produces a cur- 11 i rent of increased amplitude when there is an increase in the duration of said square-topped pulse, and means for shortng portions of said lter during fades to reduce the current output of said lter.

BERNARD M. OLIVER.

REFERENCES CITED The following references are of record'in the le of this patent:

UNITED STATES PATENTS Number Name VDate 1,585,591 Lowy May 18, 1926 2,134,716 Gunn Nov. 1, 1938 2,141,343 Campbell Dec. 27, 1938 2,189,549 Hershberger Feb. 6, 1940 2,192,189 Hacke Mar. 5, 1940 2,208,422 Hugon July 16, 1940 2,227,598 Lyman et al. Jan. 7, 1941 2,275,016 Koch Mar. 3, 1942 2,277,000 Bingley Mar. 171942 2,287,174 Heising June 23, 1942 Number Name Date 2,300,876y Dickinson Nov. 3, 1942 2,306,386 Hollywood Dec. 29, 1942 2,370,692 Shepherd Mar. 6, 1945 2,384,379 Ingram Sept. 4, 1945 2,403,527 Hershberger July 9, 1946 2,404,527 Potapenko July 23, 1946 2,412,111 Wilson Dec. 3, 1946 2,416,286 Busignies Feb. 25, 1947 2,421,018 De Rosa May 27, 1947 2,422,449 Usselman June 17, 1947 2,422,698 Miller June 24, 1947 2,442,403 Floy et al. June 1, 1948 FOREIGN PATENTS Number Country Date 474,739 Great Britain Nov. 8, 1937 497,147 Great Britain Dec. 9, 1938 OTHER REFERENCES Review of Scientific Instruments, December 1937, vol. 8, pages 502 to 504. 

